It's Dark Inside
by afaithfulwriter890
Summary: Dean has no hope - he's a demon now. A monster. Sam has given up. Castiel is the only one determined to save him, but that is easier said than done. Can Cas get to Dean and save what's left of his soul before Dean lets the darkness consume him? Destiel. Demon!Dean.
1. Preface

_It's Dark Inside - Preface_

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**Dean has no hope - he's a demon now. A monster. Sam has given up. Castiel is the only one determined to save him, but that is easier said than done. Can Cas get to Dean and save what's left of his soul before Dean let's the darkness consume him? Destiel. Demon!Dean**

**This contains an already existing Destiel, meaning that they had been in a relationship before this story takes place. **

**This story takes place after the Season 9 finale. Castiel's grace has been restored. Metatron has been defeated. Gabriel has returned and is currently ruling Heaven.**

**I hope you enjoy this fic, but just a warning, it may hurt your feels.**

**I do not own Supernatural.**

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**Prefac****e**

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_Dear Cas,_

_I'm not good with sharing my feelings – you know that. Crowley said that I shouldn't try to find you and Sam . . . that I should just stay away. And I agree with him there. I'm not going to try to find you. And you shouldn't try to find me. It's a bad idea, Cas. I'll miss you like hell, but . . . but it's better this way._

_Trust me._

_You know me better than anyone, Cas. You can see into my head. You see how I feel when no one else does. You don't judge me for it . . . you understand it and you always try to help. And you know how I think about myself. You know that I think I'm a monster, and you know that I think I deserve to die._

_Well, let's just say that that statement has even more truth to it now._

_I know you think I'm dead . . . I know Sam thinks it too._

_But I'm not._

_I'm something much worse._

_I guess my subconscious was right . . . I could never escape it. I was always going to become this. I was meant for it . . . destined for it. But I guess it makes sense._

_It all makes sense . . ._

_Look, the reason I'm sitting down and forcing myself to scribble down these meaningless words is because . . . I can't come back. I can't see you ever again. I can't see Sam. I can't see anyone. I just wanted to let you know that I was okay. But what we had, it's over. I can't . . . I can't do it. I can't . . . I can't bring myself to ever lay eyes on you again, let alone . . . let alone call you my own. I don't deserve it – I don't deserve you._

_So this is goodbye._

_I'm sorry Cas._

_I'm so fucking sorry._

_-Dean_

Castiel crumpled up Dean's letter in his hands. Tears streamed down his cheeks uncontrollably, and a sob rose in his throat. His chest ached as if someone had brought a sledgehammer down on it. His stomach had twisted into a painful knot, and all he could do was sit there in the motel room, staring at the floor. The letter slipped out of his hands and hit the floor almost silently.

Without warning, Castiel threw back his head and let out a heartbreaking cry. All the angels in Heaven, and those on earth heard Castiel's grace screaming in agony. Only a few years previous, they had heard him give a loud, triumphant shout after he had pulled Michael's vessel from hell: "Dean Winchester is saved!" Now, it was different, now his voice was stricken with pain and loss as he declared: "Dean Winchester is _dead!_"

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A/N: Review?


	2. Chapter 1

_It's Dark Inside - Chapter 1_

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**Thanks for the follows and faves!  
**

**I don't own Supernatural.**

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**Chapter 1**

Dean walked through the streets of San Francisco with his head hung. His entire body felt heavy . . . lifeless. He had accepted the Mark of Cain in hopes of saving the world – like always – and he knew the consequences if he died wearing it . . . He just thought he wouldn't die. Of course, he knew how egotistical and foolish that sounded. He had been killed countless times by now. He hadn't bothered keeping track after the whole incident when Gabriel – the archangel turned Trickster – made him die over and over again so Sam would learn a lesson, which he never even learned. Dean had just been thinking: "that would never happen to me", as so many reckless people do. And he accepted.

And now he was repaying the price.

It was bad enough that he was a demon – a monster – but there was something else that really hurting him.

Sam.

Cas.

The two people in the world he loved. Dean had lost everyone – his mother, his father, every girl he eve loved, Lisa, Ben, Bobby – except for Sam and Cas. They were all he had left – the only ones still alive – and he could never see them again. Although almost every part of him was screaming to go find them immediately, he knew better. He couldn't see them – he couldn't let them see him like this.

Dean had always considered himself to be some kind of monster. He never believed the he deserved anything good that had happened to him. He didn't deserve to have a good brother like Sam. He didn't deserve to have a wonderful boyfriend/guardian angel like Cas. He didn't deserve to have a man like Bobby to care for him. He didn't deserve to be pulled out of Hell. He didn't deserve to be treated like some kind of hero.

He was the one who started the apocalypse. He was the one who broke the first seal. He was the one who couldn't keep his brother from becoming a demon blood-junkie. He was the one who couldn't keep Sam away from Ruby. He was the one who couldn't stop Sam from killing Lilith. He was the one who said no to Michael. He was the one who almost ended the world. He was the one who couldn't protect Sammy. He was the one who couldn't keep Cas from absorbing the souls from Purgatory. He was the one who couldn't stop Metatron.

And yet he got so many good things in his life.

_I deserve this,_ he thought simply, shoving his hands into his pockets. _I deserve to be a demonic scumbag. I deserve to live on without Cas and Sam . . . What a better punishment? Alastair should have used this a form of torture._ He shuddered, thinking about the time he apprenticed under Hell's Grand Torturer. That was how he broke the first seal. That was the reason everything had gone to Hell.

Now Dean was a demon – he was no better than the son of a bitch that had killed his mother. He was no better than Ruby who had corrupted Sam and helped free Lucifer. He was no better than Lilith who had destroyed more lives than anyone was capable of comprehending.

He walked down the lonely streets – it was early in the morning, and almost no one was out. Then again, he wasn't in downtown, but one of the residential areas. He walked along the sidewalk, trying to clear his head. What was he going to do? What should he do? He didn't exactly want to be exorcised and go back to Hell – he had enough of that for a lifetime. Besides, he was in his own body – not someone else's. But he didn't understand why he was back. What was he supposed to do? Cause trouble like any other demon? Corrupt and or kill innocent humans? Fight with angels? Fight with other demons? What was his _purpose?_

"If you want a job, I can give you one, squirrel," a voice with a familiar accent sounded from behind him.

Dean whirled around to see Crowley, the King of Hell, standing there, watching with nonchalant eyes. "I wouldn't just stand there if I were you," Dean warned.

Crowley arched an eyebrow. "Oh?"

"Yeah, because if you stay there, I'm going to kill you," Dean snarled. His voice was darker and deeper than ever before. Animalistic.

Demonic.

Crowley let out a delighted laugh. "Oh, darling, you'll fit right in with the demon community," he chuckled. "I thought that your first order of business would be to track down your 'Huggy Bear', as I believe you once called him?" Crowley more asked than stated. "Either the feather-head or the moose – one of those two. So why haven't you gone to see them?"

Dean gritted his teeth. He could feel his eyes becoming wetter than he was comfortable with. The thought of them wondering what happened to him . . . Even though he sent the letter to tell Cas that he was alright, he knew that they would both be concerned for him. At least for now. They knew he was a monster now, but they didn't know he was a demon. Cas had probably put two-and-two together though, since he knew that Dean bore the Mark of Cain. He probably guessed what had happened. And he probably hated him for it.

"First of all, you told me not to. Second . . . Maybe I don't want to see them anymore," Dean said sharply. He was growing tired of Crowley's attitude. Why did he have to look down his nose at him? Why did he have to act like he was so better than him? Crowley knew _nothing_ about him.

"Oh? And why is that?"

"Because I'm a demon, in case you forgot," Dean replied. "Besides, if I went back God knows what I would end up doing to them. This freakin' demon juice inside me would probably have me go off on them, or attack them, or some bullshit like that."

Crowley shook his head. "Tsk, tsk, Dean. That's no way to act. That's your brother and you're boyfriend back there. I thought you would go after them – even if I advised you against it. The fact that you didn't concerns me."

"I have no brother," Dean said, surprised by the venom in his own voice. "And Cas and I – we are nothing."

The King of Hell just stared at him with almost stunned eyes. Dean gazed back, hard and unyielding. Crowley wanted answers – he knew that – and he was not about to give them to him. At long last, the other demon smiled. "You'll make a great weapon of mass-destruction, squirrel," he said softly. "Just keep doing what you're doing. Let the rage slowly consume you. You'll lose yourself in no time – don't worry. That little . . . human part of you that's still left – the part that's begging you to find Castiel and Sam again – it'll disappear soon. It will be lost in the darkness that is slowly engulfing your soul. And then you'll be mine."

With that, Crowley disappeared, leaving Dean standing there alone on and empty street.

"Fine," he growled, his eyes darkening to the color of obsidian. "If that's what needs to happen – then let it happen. I'll welcome the emptiness. At least then I won't have to feel anymore." That much was true. He wouldn't have to feel the self-hatred. He wouldn't have to feel the pain at losing Sam and Cas. He wouldn't have to feel guilt for leaving them like this.

He wouldn't feel anything anymore.

And he was looking forward to it.

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A/N: Review?


	3. Chapter 2

_It's Dark Inside - Chapter 2_

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**Here we get to see a little bit of how Sam views the situation, and what Cas thinks. **

**The plan at the end of the chapter - I am aware that it might be far-fetched, but just bare with me.**

**I don't own Supernatural.**

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**Chapter 2**

"We need to do something," Castiel persisted. "We have to try to find him."

Sam Winchester sat on the bed in the motel room in silence. He had lost his brother before – like when he had gone to Hell for four months all those years ago. He had lost him when Gabriel made Sam go almost a year without seeing Dean. There were plenty of times when they had been apart, but it was never anything like this. Dean was always out there – dead or alive – he was _out_ there, and he was _human._ Well, except for that time he almost became a vampire, but other than that, he was always human.

Sam knew the truth.

Sam knew something Castiel didn't.

He knew that Dean was a demon.

In Sam's mind, his brother was dead, and this time for good. There would be no getting him back. No happy reunion when Dean would return alive and healthy. Not this time. There was no happy ending to this story. There never had been a happy ending. The only reason he hadn't told Castiel all this yet – the only reason he hadn't informed him about the Mark of Cain was because he didn't want Cas to feel worse than he already did.

"And where are we going to start, Cas?" the younger Winchester asked in exasperation. "He could be anywhere."

Castiel stared at Sam in shock. Did he really her . . . reluctance in his voice? "Forgive me, but I do not understand. Do you not want to find your brother?" Sam was surprised by the harshness in Cas's voice. The angel rarely got angry, but it seemed as if his temper was beginning to boil.

"What is the point, Cas?" Sam asked.

Castiel gawked at him.

Sam sighed and shook his head, annoyed. "Dean said that we should stay away from one another, so don't you think we should listen to him?"

"No, Sam," Cas answered firmly. "I do not think we should listen to him. Dean is trying to protect us – even now as . . . whatever he is, he is looking out for us. He is afraid that if we see him like this, we'll try to hunt him, or something else of that nature. We need to find him and tell him that it is okay for him to come home with us."

"Cas, I know this is hard for you," Sam replied sharply. "But I don't think that this is a good idea. And if you want to go all over creation looking for my brother, be my guest. As for me, I am going to . . . look for a hunt – a real hunt."

The angel watched as Sam left the motel room, slamming the door behind him. Cas stood there for a minute or two after his departure, reeling. Sam had never . . . given up before, at least not like this. He had never let Dean down, especially when Dean was potentially in danger. Dean would never do that to Sam. This made Cas uneasy. What did this mean? Was Sam really . . . abandoning his brother when Dean needed him most?

Castiel roughly twisted off his trench coat and ran his hands through his hair. He had found himself doing that a lot lately. As an angel, he viewed his body as a meat-suit – just something to use. But now that he had been human, some of those human needs and tendencies had remained. Cas found himself scratching itches that he used to ignore, or feeling things that he would have otherwise ignored – hunger, thirst . . . and sickening, crippling feeling of _pain_.

The idea of Dean out there all alone . . . it made his heart ache in ways he didn't know was possible. Dean was everything to him – all he had left except for Sam. Castiel gave up everything for Dean. He pulled him from perdition. He rebelled for him. He started a civil war in Dean's name. He broke through the mind control that had almost made him kill the man he loved for Dean. He gave up his entire army for Dean. He destroyed the most powerful weapon in existence for Dean. He fell from grace for Dean. He did it _all_ for Dean. And now Dean was gone . . . Dean had _left him_.

Castiel still had a hard time wrapping his head around that fact. He could not simply imagine the idea of living without Dean Winchester. He couldn't imagine a life where he never saw Dean's face ever again. He didn't think it was _possible_ for that to ever happen. But now, it seemed to be upon him.

_No_, Cas thought. _I am not giving up on him._ Cas had to find Dean if Sam wouldn't. He had to find a way to convince him to come home. Nothing that Dean had none – nothing that he _was_ could keep Cas from caring for him.

Castiel paced back and forth across the motel room, thinking about Dean's letter. Cas had practically memorized the entire thing from start to finish. For all he knew, Dean could have encoded a secret message for him to uncover. Even though Sam had read it at least five times, and knew all of Dean's secret code words, Cas still couldn't be sure. _Maybe it wasn't intended for Sam to find . . . Maybe it was specifically for me._ It was a longshot, but Cas clung to it.

The longer he scrutinized it, the worse the pain got. He had to reread "it's over" probably a thousand times. And each time, it felt a part of himself shrivel up and die. Cas always felt a little . . . unsure of him and Dean's relationship. He didn't understand how anyone – let alone the most wonderful man he'd ever met – could love him. It might sound foolish that an _angel_ of all creatures would have low self-esteem, but that is what it seemed like. In Castiel's mind, he honestly believed that he did not deserve someone like Dean. He didn't know what the hunter saw in him. He was an angel that had fallen from grace, had been rejected by his people, and had made more unforgiveable mistakes than anyone ever should, and yet Dean still loved him. And Castiel would never question that love. He knew that it didn't make sense, but he would never downright reject it or say anything against it. Castiel respected Dean too much to ever think that he would lie about something that important. Perhaps Dean didn't feel as strongly as Castiel did, or maybe he didn't cherish it as much as Cas did, but the angel didn't doubt that the hunter felt _something_.

And yet Dean still gave it up.

That fact hurt Cas more than he thought anything ever could. He felt as if his grace was being split in two – being torn apart. Half of it was with Dean, wherever he was, and the other half was left reeling in Castiel's body.

"I have to find him," Castiel murmured. He didn't want to feel this . . . absence anymore. He didn't want to feel this gaping hole in his chest. He wanted Dean back. He wanted to see his evergreen eyes. He wanted to see his radiant smile that put the stars to shame. He wanted to feel his strong, muscular arms around his smaller form. He wanted to feel Dean's lips on his. He wanted his human back.

"I have to find him no matter what he is . . . or where he is." With that, Castiel rose to his feet. Just as he was about to take flight, the door to the motel room opened. Sam had returned.

Castiel folded up his wings and stared at the younger Winchester with narrowed eyes. "You returned," he commented.

Sam sighed. "Yeah . . . Look, Cas . . . there is something I gotta tell you."

Cas looked at the hunter with unblinking sapphire eyes. "Okay . . . I hope it is an apology. I believe you owe both me and Dean one."  
Sam ran a hand through his locks and sighed a second time. "Cas . . . there is something you don't know . . . Something Dean didn't tell you."

Immediately Cas felt his gut clench. It was a human reaction, but one he had associated with fear, or terror. He gulped and nodded for Sam to continue.

"Before Dean was killed he . . . accepted the Mark of Cain."  
"WHAT?!" Castiel nearly roared.

Sam immediately recoiled. He had never seen the angel lose control like that, let alone to such a magnitude. It was as if a volcano had erupted. "WHAT DO YOU MEAN HE ACCEPTED THE MARK OF CAIN?!" he snarled. "DID HE HAVE ANY IDEA WHAT THAT MEANT FOR HIM?! DID HE KNOW WHAT THAT WOULD DO?!"  
"Y- yes . . ." Sam whimpered in response. "He knew, Cas . . . He knew and he accepted it anyway. It was the only chance we had to defeat Metatron. He had no choice – he did what he thought was best."

Cas began to pace back and forth, fuming. Now he understood. He knew why Dean was staying away. He knew why he had called himself a monster. He knew why Sam had _given up._

Dean wasn't just an ordinary monster.

He was a demon.

"Damn you, Cain," Cas hissed under his breath. "I hope you rot in the deepest pit of hell for the next ten thousand years!" he roared the last part so loud that he was almost certain that Cain, wherever he was, had heard him.

"We need to find a way to change him back," Castiel said after a few moments, his voice softer now, and more thoughtful. He felt the rage begin to ebb away. _Focus,_ he tried to tell himself. If he wasn't focused on the task at hand, he wouldn't be able to come up with a solution.

"And how exactly are we going to do that?" Sam countered. "As far as we know, there is nothing that can turn a demon back into a human."

"But this isn't an ordinary case," Castiel pointed out. "Demons are created when human souls go to hell. Slowly but surely, their humanity is burned away. It can take decades, centuries – even a millennium if the human is strong-willed. But the point is: the humanity inside them is destroyed by fire, torture, and rage. This is not the case with Dean. He only became a demon because of the mark. That means that some of his humanity might remain intact."

Sam looked a little lost. "So . . . My brother is only . . . half demon?"

"Perhaps," Cas answered. He was brainstorming, and becoming more and more excited by the minute. They might have a chance. He might still be able to see Dean again – be able to save him. "If some of his humanity remains . . . It might be able to drive out the demon part of him."

"Like how I subdued Lucifer?"

"Yes."

Now Sam was beginning to understand the plan. Castiel saw some light return to Sam's eyes, as if he was beginning to see the possibility of saving his brother. "So there is actually a chance?"

"If it's not too late . . . I don't know how . . . demonic Dean actually is at this stage. Dean might be more human than demon, he might be more demon than human, or he might be all demon. I don't know. But we have to find him – try to find out, and try to help."

Sam took a deep breath and nodded. Castiel was relieved by the look of determination on his face. "Okay. Where do we start?"

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A/N: **Please remember to review! They are what keeps me writing!**


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